Cancer has become a leading cause of death worldwide, accounting for over 7.5 million deaths in 2008. While one of the most important forms of cancer treatment, chemotherapeutic drugs often also kill healthy cells and cause toxicity to the patient. In the search for improved alternatives nanocarriers have become an emerging platform for cancer therapy enabling drug delivery specifically into tumors. Beginning in the mid-1980s, several types of targeting nanocarriers, based on polymer-protein conjugates and lipids, have successfully reached clinical trials. However, there are still many challenges remaining, including rapid clearance, burst drug release and non-specific uptake. In order to overcome these challenges, nanocarriers with better properties need to be designed. A promising alternative material to polymers is mesoporous silica due to its high-surface area, stability and bio-compatibility. Surface-functionalized mesoporous silica nanoparticles (MSNs) can deliver multiple types of cargo, such as DNA, drug molecules or even quantum dots, into cells and tissues of plants or animals. However, in the current state of development such MSN-based nanocarriers have not yet reached clinical trials. One of the reasons is that silica typically needs a fairly long time to dissolve under physiological conditions, resulting in potential particle accumulation in the body, which may in turn cause long-term toxicity. Even in cases where MSNs dissolve quite rapidly, questions about the dissolution mechanism, bio-distribution and toxicity remain.
In order to endow ultra-small silica nanoparticles with additional, e.g. therapeutic properties for clinical applications it is desirable to develop mesoporous particles with sizes smaller than 10 nm. Although recently the size of MSNs has been pushed down to less than 20 nm, the synthesis of fluorescent MSNs smaller than 10 nm and with narrow particle size distributions still remains a challenge.